Generally, fine pattern formation is carried out by photolithography in the manufacture of a semiconductor device. A number of substrates called transfer masks are normally used for such fine pattern formation. The transfer mask comprises generally a transparent glass substrate having thereon a fine pattern made of a metal thin film or the like. The photolithography is used also in the manufacture of the transfer mask.
In the manufacture of a transfer mask by photolithography, use is made of a mask blank having a thin film (e.g. a light-shielding film) for forming a transfer pattern (mask pattern) on a transparent substrate such as a glass substrate. The manufacture of the transfer mask using the mask blank comprises an exposure process of writing a required pattern on a resist film formed on the mask blank, a developing process of developing the resist film to form a resist pattern in accordance with the written pattern, an etching process of etching the thin film along the resist pattern, and a process of stripping and removing the remaining resist pattern. In the developing process, a developer is supplied after writing the required pattern on the resist film formed on the mask blank to dissolve a portion of the resist film soluble in the developer, thereby forming the resist pattern. In the etching process, using the resist pattern as a mask, an exposed portion of the thin film, where the resist pattern is not formed, is removed by dry etching or wet etching, thereby forming a required mask pattern on the transparent substrate. In this manner, the transfer mask is produced.
For miniaturization of a pattern of a semiconductor device, it is necessary to shorten the wavelength of exposure light for use in photolithography in addition to miniaturization of the mask pattern of the transfer mask. In recent years, the wavelength of exposure light for use in the manufacture of a semiconductor device has been shortened from KrF excimer laser light (wavelength: 248 nm) to ArF excimer laser light (wavelength: 193 nm).
As a type of transfer mask, a halftone phase shift mask is known apart from a conventional binary mask having a light-shielding film pattern made of a chromium-based material or the like on a transparent substrate. This halftone phase shift mask is configured to have a phase shift film on a transparent substrate. This phase shift film is made of, for example, a material containing a molybdenum silicide compound and is adapted to transmit light having an intensity that does not substantially contribute to exposure (e.g. 1% to 20% at an exposure wavelength) and to produce a predetermined phase difference. By the use of light-semitransmissive portions formed by patterning the phase shift film and light-transmissive portions formed with no phase shift film and adapted to transmit light having an intensity that substantially contributes to exposure, the halftone phase shift mask causes the phase of the light transmitted through the light-semitransmissive portions to be substantially inverted with respect to that of the light transmitted through the light-transmissive portions so that the lights having passed near the boundaries between the light-semitransmissive portions and the light-transmissive portions and bent into the others' regions due to diffraction cancel each other out. This makes the light intensity at the boundaries approximately zero to thereby improve the contrast, i.e. the resolution, at the boundaries.
With respect to the transfer mask and the mask blank, the miniaturization of the mask pattern of the transfer mask requires a reduction in thickness of the resist film formed on the mask blank and dry etching as a patterning technique in the manufacture of the transfer mask.
However, the reduction in thickness of the resist film and the dry etching have the following technical problems.
One problem is that, for example, the processing time of the light-shielding film exists as one serious restriction to the reduction in thickness of the resist film on the mask blank. Chromium is generally used as a material of the light-shielding film and, in dry etching of chromium, a mixed gas of chlorine gas and oxygen gas is used as an etching gas. When patterning the light-shielding film by dry etching using the resist pattern as a mask, since the resist film is an organic film composed mainly of carbon, it is very weak against an oxygen plasma forming a dry etching environment. While patterning the light-shielding film by dry etching, the resist pattern formed on the light-shielding film should remain with a sufficient thickness. As one index, in order to make excellent the cross-sectional shape of the mask pattern, the resist film is required to have a thickness that still remains even when the etching time is about twice a just etching time (100% overetching). For example, since, in general, the etching selectivity of chromium as the material of the light-shielding film to the resist film is 1 or less, the thickness of the resist film is required to be twice or more that of the light-shielding film. Therefore, it is necessary to shorten the processing time of the light-shielding film for reducing the thickness of the resist film and, for that purpose, it is important to reduce the thickness of the light-shielding film. However, while reducing the thickness of the light-shielding film, the light-shielding film is required to have a predetermined optical density (normally 3.0 or more at a wavelength of exposure light for use with a mask) for ensuring its light-shielding performance and therefore the reduction in thickness of the light-shielding film has its own limitation.
In view of this, as a method of reducing the thickness of the resist film, there has been proposed a method of forming, on a light-shielding film, an etching mask film made of a material to which the etching selectivity of the light-shielding film is high, then etching the etching mask film using a resist pattern as a mask to form an etching mask film pattern, and then etching the light-shielding film using the etching mask film pattern as a mask to form a light-shielding film pattern (see JP-A-2006-146152 (Patent Document 1)). In Patent Document 1, an oxynitride of silicon or the like is proposed as a material of the etching mask film.